The problem is that there's too much competition for the other slots. Compared to DPS classes, healers have more mandatory cross class to take such as Protect, Lucid Dream (for MP), Swiftcast, Esuna (if the battle requires something to be dispelled). And then we have Largesse that is better than every other cross class we have and there's no choice at all. Most of the time we will pick the same. Probably Cleric for DPS increase but still.